The present invention generally relates to methods and apparatus for controlling the tension in a zone between two points along a web, tape or strand of material. In particular, the invention relates to methods and apparatus for controlling the tension in continuous plastic material being fed into a packaging machine.
There are in existence many devices for controlling tension in a web, tape or strand of material and, in particular, in a moving web, tape or strand as it is unwound from a roll or spool, moves through, over, around, and between various feed rolls and, ultimately is rewound onto a take-up roll or spool or is otherwise processed. There are numerous types of systems that require tension control devices in order for the process to be carried out satisfactorily and such that the web, tape or strand is not strained to an undesirable degree. Typical of applications and systems where tension control is required are printing applications, plastic and other film forming and extruding operations, various processing applications, weaving applications, wire drawing applications, film and tape winding, and many other applications.
Many such applications have a payout roll or spool from which material is drawn. As more material is drawn off, the effective diameter of the roll and the roll inertia change. Many such applications also include take-up or rewind rolls or spools onto which the material is rewound, and in which the effective roll diameter and roll inertia increase as the operation proceeds. Between the payout roll and the rewind roll may be any number of other rolls or pairs of rolls around which and between which the material moves. In order to maintain optimal operating conditions, the tension in the material being processed may need to be controlled within specified limits. The characteristics of the material involved, as well as of the process, will determine the most desirable tension and how much variation in tension can be tolerated. It is also extremely important in many applications that wide variations in tension and sudden sharp tension changes or shocks be avoided to prevent damage and breakage in the material.
The need for tension control is critical in packaging systems that require precise registration of a slider-zipper assembly relative to a web of packaging film that is unwound from a supply reel and advanced intermittently. For example, in the case of a thermoforming packaging machine that thermoforms a succession of pockets in an intermittently advancing web of film and then attaches a zipper material having sliders and slider end stop structures spaced therealong, it is critical that the slider end stop structures be in proper registration with the successive pockets in the web. After the package has been filled and sealed, the web and zipper will be cut along a transverse line to sever a finished package from the remainder of the web with attached zipper material. The slider end stop structure on the zipper in registration with a web section spanning successive thermoformed pockets will be bisected by the transverse cut. A loss of registration can result in misalignment of the center of the end stop structure with the transverse cutting line, which could result in production of defective packages, e.g., packages in which the slider can be readily pulled off the end of the zipper.
In conventional tension control schemes used in thermoforming packaging machines with slider-zipper assembly application, the zipper process pathway typically passes through a combination of servo motors and tension dancers on its way to the packaging machine. The motion and reaction of these devices must be coordinated with the operation of the downstream equipment in order to maintain accurate tension and registration. Such registration and tension control schemes are relatively complex and costly to install, and must be tuned to the stroke of the packaging machine. Conventional control schemes rely on combinations of servo motors and tension dancers, and the motion and reaction of these devices must be coordinated with the downstream equipment in order to maintain accurate tension and registration. Control is provided by a costly servo controller and intensive PLC-based system. These control schemes are usually more costly and more complex to tune and maintain in proper operation.
There is a need for a simple, inexpensive and accurate scheme for controlling the tension and registration of one material (e.g., plastic zipper) having attachments or formed features, as it is fed to a sealing station, where it is joined to and later pulled by another material (e.g., packaging film) also having formed features. The tension control equipment should be easy to install. Also, the scheme for controlling tension in the pulled material should be adaptable to machines in which each advance of the pulling material in the packaging machine is equal in distance to multiple package lengths, while a trailing portion of the pulled material, upstream of the packaging machine, advances in increments of one package length.